Asphalt shingles are normally finished with colored granules of mineral material that are deposited by gravity on the tacky surface of a continuous belt of roofing material, the granules being deposited in for example four different drops and a backfall to fill in the voids between the granule patterns from the four drops that give the shingles their color characteristics. Each of the drops and the backfall consist of a different mix, each of which mixes may consist of different percentages of granules of up to five different colors. The present invention relates to apparatus for preparing such mixes.
Mixes of granules have generally been prepared by metering the granules by gravity from individual silos each containing a selected color onto a moving mixing belt. Normally, each silo is fitted with a calibrated orifice plate that, when moved to its opened position, will provide an orifice of the size required to provide the desired flow rate for the granules from the silo onto the mixing belt. Systems of this nature have been operated manually whereby, after all of the selected silos for a particular mix have been fitted with the appropriate orifice plates, and after the belt has been operated for a period of time sufficient to purge the system of granules from a previous mix that would contaminate the current mix, the operator moves the orifice plate to open the outlet of the first silo to be used in the mix. Then, walking along with the belt at the speed of the belt, the operator successively opens the outlets of the other selected silos in the same manner--each of the outlets being opened as the point on the belt at which the mix begins, that is, the point that was directly below the outlet of the first silo when the outlet of that silo was opened, passes successively beneath the outlets of the other silos. The mix is delivered by the mixing belt to a bin from which it is later deposited by a blending apparatus onto a belt of asphalt material from which the shingles are cut.
Pneumatic systems have been proposed in which an operator at a central location can control the flow of the granules from the silos. These systems, which may generally parallel the manual system but with preselected orifice plates or valves opened and closed by air cylinders or with the amount of the valve opening controlled by air cylinders, have not been found to be satisfactory. Relatively minor variations in the flow rate of the colors in the mix can produce noticeable variations in the product--which results in waste or customer complaints. In addition to being subject to operator error, it is difficult to maintain pneumatic systems and to correct malfunctions due to wet or dirty air that can clog or freeze the air lines or require recalibration of the instruments.